1. Field of the Invention
The present invention relates generally to a control message transmitting/receiving method in a mobile communication system, and in particular, to a method of transmitting/receiving a control message between a radio network controller (RNC) and a user equipment (UE) in a mobile communication system providing multimedia broadcast/multicast service (MBMS).
2. Description of the Related Art
Owing to today's development in the telecommunications industry, CDMA (Code Division Multiple Access) mobile communication systems have evolved from voice service to multicast multimedia communications which enable transmission of a large volume of data such as packet data and circuit data. Hence broadcast/multicast service is being actively developed in which one data source serves a plurality of UEs to support the multicast multimedia communications. The broadcast/multicast service is categorized into cell broadcast service (CBS) being a message-centered service and MBMS supporting multimedia data such as real-time pictures and voice, still images, text, etc.
A network configuration for providing the MBMS in a mobile communication system will be described with reference to FIG. 1.
FIG. 1 is a schematic view illustrating the configuration of a network for providing an MBMS service in a mobile communication system.
Referring to FIG. 1, a multicast/broadcast-service center (MB-SC) 110 serves as an MBMS stream source. It schedules MBMS streams and transmits the streams to a transit network (NW) 120. The transit NW 120, located between the MB-SC 110 and an SGSN (Serving GPRS Support Node) 130, transfers the received MBMS streams to the SGSN 130. The SGSN 130 can be configured with a GGSN (Gateway GPRS Support Node) and an external network. It is assumed here that a plurality of UEs, UE 1 161, UE 2 162, UE 3 163 within Node B 1 (i.e. cell 1) 160, and UE 4 171 and UE 5 172 within Node B 2 (i.e. cell 2) 170 are to receive the MBMS service. The SGSN 130 controls MBMS-related services for the UEs, such as management of MBMS-related billing data and selective transmission of MSMS service data to a particular RNC 140. For simplicity, Node B is used herein to describe the cell itself. Obviously, a Node B manages one or more cells.
The SGSN 130 selectively transmits MBMS service data to the RNC 140 and the RNC 140 selectively transmits the MBMS service data to cells. For the selective transmission, the SGSN 130 must know which RNCs are to receive the MBMS service data, including RNC 140, as well as which cells are to receive the MBMS service data. Thus, the RNC 140 can provide the MBMS service to the cells. The RNC 140 controls a plurality of cells, transmits MBMS service data to cells having UEs requesting the MBMS service, controls radio channels established for providing the MBMS service, and manages MBMS-related information using MBMS streams received from the SGSN 130. As illustrated in FIG. 1, one radio channel is established for the MBMS service between a Node B and UEs within the coverage area of the Node B, for example, between cell 2 170 and UEs 171 and 172. An HLR (Home Location Register: not shown) is connected to the SGSN 130 and authenticates MBMS subscribers.
To provide a specific MBMS service, basic information about the MBMS service is provided to UEs. If the UEs want to receive the MBMS service, a list of the UEs is transmitted to a network. The network then pages the UEs and establishes radio bearers (RBs) for the MBMS service. Thus, the MBMS service is provided to the UEs through the RBs. If the MBMS service is terminated, the UEs are informed of the MBMS service termination and release all resources assigned for the MBMS service. This is a normal MBMS service procedure.
FIG. 2 is a diagram illustrating a signal flow for providing an MBMS service between a UE and a network in the mobile communication system
Referring to FIG. 2, the UE subscribes to the MBMS service through a core network (CN) in step 201 (subscription). The CN includes an MB-SC, a transit NW, and an SGSN as illustrated in FIG. 1. The subscription is a process of exchanging basic information related to MBMS billing or MBMS reception between a service provider and a user. When the subscription is completed, the CN notifies UEs of current available MBMS services along with their basic information by, for example, menu information, in step 202 (announcement). The menu information contains the times and durations of the MBMS services. The CN broadcasts the menu information as a general announcement, for example, by CBS, or transmits it only to UEs requesting MBMS services. The CN also notifies the UEs of service IDs identifying the respective MBMS services by the menu information.
Upon receipt of the menu information in step 202, the UE selects an intended MBMS service from the menu information and transmits a service request message to the CN in step 203 (joining). The service request message includes the ID of the selected MBMS service and the ID of the UE. The CN then identifies the requested MBMS service and establishes a multicast mode bearer for the UE in step 204 (multicast mode bearer setup). During the multicast mode bearer setup, transport bearers can be set up beforehand over the CN, that is, between the SGSN and the transit NW. For example, a GTP-U/UDP/IP/L2/L1 bearer (refer to 3GPP TS 23.060) can be established ahead of time between the SGSN and a GGSN. The CN then notifies the UE that the requested MBMS service will start soon, through a paging type notification, in step 205 (notification). The paging can be carried out conventionally or in an optimized paging method for MBMS as disclosed in Korea Patent Application No. 2002-34704 filed by the same applicant. The UE is then actually assigned the radio resources necessary for the MBMS service in a radio resource allocation procedure with the CN, and implements the assigned radio resources in hardware in step 206 (radio resource allocation). The radio resource allocation occurs in two steps, a step for the RNC notifying UEs within an arbitrary cell of information about an RB established for the MBMS service in the cell (hereinafter, referred to as radio bearer setup), and a step for the RNC transmitting to cells having UEs requesting the MBMS service information about transport bearers and radio bears to be set up on lub interfaces (hereinafter referred to as radio link setup). The RB setup will be described later with reference to FIG. 4. When the radio resource allocation is completed, all UEs that have requested the MBMS service are informed of the radio links on which the MBMS service is provided, and of higher layers in which the MBMS service is processed. The cells of the UEs completely establish the radio links and lub interfaces. With the preparation for the MBMS service completed between the RNC and the UEs, the CN transmits MBMS service data to the UEs through the RNC in step 207 (data transfer). In step 208, the radio resources, that is, the transport bearers and radio bearers are released between the UEs and the CN when the MBMS data transmission is completed (radio resource release).
Steps 203 to 206 illustrated in FIG. 2 will be described in more detail with reference to FIG. 3. Although the CN generically refers to the SGSN 130, the transit NW 120, and the MB-SC 110, only the SGSN 130 will be considered below in connection with the operation of the RNC 140.
FIG. 3 is a diagram illustrating a detailed signal flow for steps 203 to 206 depicted in FIG. 2
Referring to FIG. 3, the UE 161, after receiving basic information about a specific MBMS service, in step 202, transmits an ACTIVATE MBMS PDP (Packet Data Protocol) CONTEXT REQUEST message to the SGSN 130 in a CELL_FACH state in step 301. Here, a PDP context includes a primary PDP context and a secondary PDP context. The second PDP context exists only if the primary PDP context exists. It has the same information as the primary PDP context, but utilizes a different GPRS (General Packet Radio Service) GTP (GPRS Tunneling Protocol) tunnel. The GPRS is a packet data service deployed in a UMTS network. The ACTIVATE MBMS PDP CONTEXT REQUEST message includes the parameters of NSAPI (Network layer Service Access Point Identifier), TI, PDP type, PDP address, access point network, and QoS (Quality of Service). The mobile communication system creates a GTP tunnel to the SGSN 130 in the case where the UE 161 requests it (i.e. UE-initiated activate), or it to the CN in the case where an external network requests (i.e. network-requested activate).
Upon receipt of the ACTIVATE MBMS PDP CONTEXT REQUEST message, the SGSN 130 creates an MBMS PDP context for the MBMS service if the UE 161 is the first one to request the MBMS service, stores information about the UE 161 in the MBMS PDP context, and performs a predetermined operation in conjunction with a GGSN connected to the SGSN 130. This operation is about GTP tunneling. When the SGSN 130 notifies the GGSN of the parameters set in the ACTIVATE MBMS PDP CONTEXT REQUEST message, the GGSN sets up a GTP tunnel based on the parameters. The MBMS PDP context is a set of variables containing information about the MBMS service. It includes a list of UEs that have transmitted the ACTIVATE MBMS PDP CONTEXT REQUEST message, the locations of the UEs, and transport bearers by which the MBMS service data is transmitted. The SGSN 130 then transmits to the UE 161 an ACTIVATE MBMS PDP CONTEXT REQUEST ACCEPT message in step 302. This message contains TMGI (Temporary Multicast Group Identity) for group paging in connection with the MBMS service, and DRX (Discontinuous Reception). The DRX is related to a cycle in which the UE 161 monitors a PICH (Paging Indicator Channel). The DRX contains a DRX CL (Cycle Length) coefficient and an Np. The Np represents the number of paging instances (PIs) in one system frame and is given as system information (SI). Its value is one of [18, 36, 72, 144]. The uses of the TMGI and DRX are disclosed in Korea Patent Application No. 2002-34704 filed by the same applicant. Upon receipt of the ACTIVATE MBMS PDP CONTEXT ACCEPT message, the UE 161 transitions to an idle state. Meanwhile, the SGSN 130 transmits a NOTIFICATION message to the RNC 140 to which the UE 161 belongs when the MBMS service is about to start or when the SGSN 130 receives first MBMS service data from the MB-SC 110, in step 303. Since the SGSN 130 stores a list of UEs requesting the MBMS service, and the RNCs to which they belong, the SGSN 130 transmits the NOTIFICATION message to the RNCs when the MBMS service is initiated. The NOTIFICATION message contains the TMGI and DRX.
Upon receipt of the NOTIFICATION message, the RNC 140 performs step 304. Specifically, the RNC 140 calculates a paging occasion (PO) and a PI using the TMGI and DRX. In the same manner, the UE 161 calculates the PO and PI using the TMGI and DRX contained in the ACTIVATE MBMS PDP CONTEXT REQUEST ACCEPT message. The RNC 140 informs the UE 161 of whether it will receive a PCH (Paging Channel) by setting the PICH to on or off state at a time point indicated by the PI and PO. If the PICH is on in the PI of the PO, the UE 161 receives the PCH signal and recognizes that it is paged. On the contrary, if the PICH is off, the UE 161 does not receive the PCH. Meanwhile, if the UE 161 is paged, the RNC 140 transmits to the UE 161 the NOTIFICATION message or a paging message on a PCH associated with the PICH a predetermined time after the transmission of the PICH, so that the UE 161 can be informed that the MBS service will start soon or it will receive the NOTIFICATION message or the paging message. The NOTIFICATION message is a type of paging message, containing information about message type, paging cause, and TMGI. The paging cause indicates the reason for the paging. In the current W-CDMA mobile communication system, “terminating streaming call” is defined as a paging cause for MBMS. Aside from the existing paging cause, a novel paging cause can be defined for MBMS. For simplicity, the NOTIFICATION message or paging message will be hereinafter referred to as “an MBMS paging message.”
In the mean time, the UE 161 monitors the PICH at the PI of the PO. It receives an MBMS paging message on an associated PCH if the PICH is on, and does not receive it if the PICH is off. When ‘1’ is coded in the PI of the PO, it means that the PICH is on. On the other hand, when ‘0’ is coded in the PI of the PO, it means that the PICH is off. Upon receipt of the MBMS paging message, the UE 161 determines which MBMS service will be initiated based on the TMGI contained in the MBMS paging message. If the TMGI indicates the MBMS service that the UE 161 has requested, the UE 161 awaits for receipt of corresponding MBMS service data.
After receiving the MBMS paging message, the UE 161 transitions to the CELL_FACH state and transmits to the SGSN 130 a NOTIFICATION RESPONSE message indicating normal reception of the NOTIFICATION message in step 305. The SGSN 130 transmits to the RNC 140 an MBMS RAB (Radio Access Bearer) ASSIGNMENT REQUEST message in step 306. The MBMS RAB ASSIGNMENT REQUEST message may contain QoS and a list of UEs for which an MBMS RAB is to be set. While the description centers on the UE 161, if a plurality of UEs request the MBMS service, the MBMS RAB ASSIGNMENT REQUEST message, including a list of the UEs, is delivered to the RNC 140. The RNC 140 then performs a preset operation required to provide the MBMS service to the UEs. RAB is a set of transmission resources configured in an RNC to provide the MBMS service. Specifically, the RAB includes a transport bearer on the lub interface between the SGSN 130 and the RNC 140, a transport bearer on the lub interface between the RNC 140 and the Node B 160, and radio channels.
The RNC 140 determines MBMS RB information (MBMS RB info) about the MBMS service in relation to the MBMS RAB ASSIGNMENT REQUEST message. The MBMS RB info covers layer 2 (L2) information and layer 1 (L1) information. The L2 information can be RLC (Radio Link Control)/PDCP (Packet Data Convergence Protocol)-related information. The L1 information may include information about TFS (Transport Format Set), TFCS (Transport Format Combination Set), channelization code, and transmit power. The RNC 140 determines cells for which the MBMS RAB is established according to the list of UEs. Since it perceives the locations of UEs in the CEL_FACH state by cells, the RNC 140 can translate the UE list into a list of cells. Thus, the RNC 140 transmits the MBMS RB SETUP message to the individual cells as many times as the number of the cells.
In step 307, the RNC 140 transmits to the UE 161 the MBMS RB SETUP message. The UE 161 then sets up an MBMS RB according to the MB RB info and transmits an MBMS RB SETUP COMPLETE message to the RNC 140 in step 308. The RNC 140 transmits an MBMS RAB ASSIGNMENT RESPONSE message to the SGSN 130 in step 309. The SGSN 130 then starts to transmit MBMS service data to the UE 161 in step 207.
The messages NOTIFICATION and MBMS RB SETUP illustrated in FIG. 3 are group messages. A group message is defined as a message transmitted commonly to a plurality of UEs. That is, the UEs decide whether they are to receive the NOTIFICATION message on the PICH, referring to the same PI of the same PO in step 304. Since the TMGI indicates the UEs to receive the NOTIFICATION message, they can receive this message. Also, the MBMS RB SETUP message with the TMGI inserted therein is transmitted commonly to the UEs on an FACH (Forward Access Channel).
FIG. 4 illustrates steps 307 and 308 depicted in FIG. 3 in more detail before describing FIG. 4, it is to be appreciated that the RNC 140 manages cells 160 and 170 and it is assumed that n UEs including the UEs 161 and 162 within cell 160 request the same MBMS service. It is also to be noted that like reference numerals denote the same steps shown in FIG. 3.
Referring to FIG. 4, the RNC 140 receives the MBMS RAB ASSIGNMENT REQUEST message from the SGSN 130 in step 306. The RNC 140 then broadcasts the MBMS RB SETUP message to the n UEs in step 401. The MBMS RB SETUP message contains the MBMS RB info and an RRC state indicator. The RRC state indicator is set to indicate transition to a CELL_PCH state in the case of complete transmission of control messages between the RNC 140 and the n UEs (RRC state indicator=CELL_PCH). The MBMS RB SETUP message is transmitted to cells on an FACH and thus the UEs in the CELL_FACH can receive the MBMS RB SETUP message. Hence, the MBMS RB SETUP message functions to provide common MBMS RB info within one cell. Therefore, common transmission of the MBMS RB SETUP message to the UEs by their cell is preferred to transmission of the MBMS RB SETUP message to the individual UEs. Therefore, the use of a broadcasting channel defined as the FACH enables broadcasting of the MBMS RB SETUP message.
Each of the n UEs transmits the MBMS RB SETUP COMPLETE message to the RNC 140 and transitions to the CELL_PCH state because RRC state indicator=CELL_PCH in steps 402-1 to 402-n. 
Meanwhile, the RNC 140 transmits to the SGSN 130 the MBMS RAB ASSIGNMENT RESPONSE message in response to the MBMS RAB ASSIGNMENT REQUEST message in step 309.
In the above procedure, each of the UEs can transmit the MBMS RB SETUP COMPLETE message on an RACH (Random Access Channel). However, due to the limited capacity of the RACH, if a plurality of UEs attempt to transmit the MBMS RB SETUP COMPLETE message at the same time, system performance may be severely degraded. As illustrated in FIG. 4, since the UEs each transmit the MBMS RB SETUP COMPLETE message when step 401 is almost completed, it can be said that the UEs transmit the MBMS RB SETUP COMPLETE message simultaneously. The resulting congestion of the MBMS RB SETUP COMPLETE message traffic leads to degraded system performance.